If Sanjay can paint a house in 15 hours working alone, how long will it take to paint the house if Mohit helps him?
(A) Mohit can paint the house in 20 hours working alone.
(B) Working together with Sanjay, Mohit does 3/7 of the total work.

A) If Mohit takes 20 hours, 

Both of them took = 15*20 / 15+20 = 300/35 hours

So, this statement is enough to answer the question.

B) Since number of hours is inversely proportional to amount of work done.

So, Since Sanjay's working hours is given and Mohit's amount of work is given, we can find out the no. of hours taken by Mohit.

Work ratio of Sanjay and Mohit = 4:3

No. of hours required ratio = 15:x

So, inverse ratio

4/3 = x/15

x = 20

Hence, we can find number of hours taken by Mohit from individual sentences.